1) Field of the Invention
The present invention relates to an optical waveguide that is used for optical communication. Particularly, the present invention relates to an optical waveguide of a diffusion type waveguide capable of reducing a curvature of a curved portion and capable of making an optical device small, an optical device that uses the optical waveguide, and a method of manufacturing the optical waveguide.
2) Description of the Related Art
An optical device formed with an optical waveguide is provided using an electrooptic crystal such as an LN substrate (LiNbO3: lithium niobate), forming a metal film such as titanium (Ti) on a part of the crystal substrate, and thermally diffusing the metal film. To connect a plurality of optical devices each formed with the optical waveguide, a curved optical waveguide is formed on a part of the optical waveguide. Based on the provision of the curved optical waveguide, the direction of a straight optical waveguide can be turned. Therefore, it becomes possible to make the optical device small, and, particularly, make a size in its length direction small. An apparatus on which the optical waveguide is mounted can also be provided in compact.
When a curved waveguide is formed on a part of the optical waveguide, an optical loss (i.e., a radiation loss) that occurs at the curved waveguide portion becomes a problem. In order to suppress the loss at the curved waveguide portion, a technique is so far proposed to combine light that is radiated at the curved optical waveguide portion again on the optical waveguide, by providing a reflector on the external periphery of the curved optical waveguide (see Japanese Patent Application Laid-open No. H11-167032, for example).
One example of the optical device that is configured using the optical waveguide will be explained. FIG. 16 is a top plan view of an optical modulator that illustrates an example of the conventional optical device. This optical modulator 20 is a Mach-Zehnder interruption type optical modulator, and has a straight optical waveguide. A data line 22 that is supplied with an electronic signal such as DC is formed to have a predetermined length L0 in its longitudinal direction on a substrate 21. Both ends of the data line 22 are led out to one side of the substrate 21 respectively, and signal electrodes 22a and 22b are formed. A ground electrode 23 is formed on other partition of the substrate 21 excluding the data line 22. An optical waveguide 24 is disposed linearly along the data line 22, and two optical waveguides 24 are disposed in parallel between branch portions 24a and 24b. 
When the electronic signal is supplied to the data line 22 and also when the optical signal is supplied to the two optical waveguides 24, a phase difference due to mutual interference can be generated between the two optical waveguides 24 in a state that a speed of a progressive wave of the electronic signal (microwave) that is supplied to the data line 22 and a speed of an optical wave that flows through the two optical waveguides 24 are matched. In order to generate the phase difference, a predetermined work length (corresponding to the length L0) is necessary as a length of the parallel two optical waveguides 24. A few centimeters are necessary for the length of L0.
However, even when the conventional technique is used, the curvature of the curved optical waveguide cannot be set small. For example, when the curvature of the curved optical waveguide is set small to a few millimeters, the recombining of the light becomes insufficient, and the optical loss increases remarkably. When the curvature of the curved optical waveguide cannot be set small, the total optical waveguide cannot be made small, which makes it impossible to make small the optical device that is formed with the optical waveguide or to integrate the optical device. For example, even when a curved optical waveguide is formed on a part of the optical waveguide of the optical device as shown in FIG. 16, the optical device cannot be made compact.
The present invention has been achieved in order to solve the above problems, and has an object of providing a compact optical waveguide, a compact optical device, and a method of manufacturing the optical waveguide capable of suppressing the optical loss.